Inorganic-organic hybrid thin-film transistors using inorganic semiconducting films

ABSTRACT

Inorganic semiconducting compounds, composites and compositions thereof, and related device structures.

This invention was made with government support under Grant No.CHE-0201767 awarded by the National Science Foundation, Grant No.DMR-0076097 awarded by the National Science Foundation, Grant No.NCC-2-1363 awarded by the National Aeronautics and Space Administration(NASA), and Grant No. W911NF-05-1-0187 awarded by the Army ResearchOffice (DARPA). The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

Thin-film transistors (TFTs) pervade our daily lives as indispensableelements in a myriad of electronic/photonic products, such as computers,cell phones, displays, household appliances and sensors. Furthermore,the future demand for next-generation mobile computing, communicationand identification devices is expected to increase markedly. For diversemultiple functionalities, the electronics of ideal mobile devices mustachieve light weight, low power consumption, low operating voltages(powered by household batteries) and compatibility with diversesubstrates. Additional desirable features include optical transparency(‘invisible electronics’), mechanical ruggedness, environmentalstability and inexpensive room-temperature/large-area fabrication.

TFTs meeting all the aforementioned requirements have proved elusive andwill doubtless require a new direction in choice of materials andprocessing strategies. Conventional inorganic TFTs based on silicon andrelated semiconductors exhibit desireable features, such as high carriermobilities, but are also limited by marginal mechanical flexibilityand/or mandatory high-temperature processing (frequently>400° C. forII/VI and III/V compound semiconductors and >250° C. for Si TFTs). Whileamorphous silicon TFTs have been fabricated on flexible plasticsubstrates at temperatures as low as 75-150° C., reported carriermobilities are modest (˜0.03-1 cm² V⁻¹ s⁻¹ on inorganic insulators) andthe material is optically opaque. Organic semiconductor materialsprovide low temperature processability and are compatible with substrateflexibility, but have typically provided low field-effect mobilities.Likewise, various concerns persist relating to choice of dielectricmaterial and corresponding fabrication technique, such concerns as canrelate to choice and incorporation of any one particular semiconductormaterial. Such complexities and competing issues illustrate an on-goingconcern in the art. The search continues for a comprehensive approach toTFT fabrication, one available at low process temperatures and/orcompatible with flexible plastic substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-D. Schematic views of TFTs using thin-film metal oxidesemiconductors as a channel, showing In₂O₃ as one such possiblesemiconductor material: (A) TFTs on doped-Si gate substrates; thedielectrics include 300 nm thermally-grown SiO₂, SAS nanodielectrics(iteratively applied, where n can be, e.g., 3), and cross-linked polymerdielectrics; (B) transparent flexible TFTs using a polymer blenddielectric (see also FIG. 1G), on PET/ITO substrates; (C) TFTs, L(channel length)=50/100 μm, W (channel width)=5 mm, on doped Si gatesubstrates (left): the dielectrics are 300 nm thermally grown SiO₂, a16.5 nm self-assembled SAS dielectric or a 20 nm CPB dielectric, anddrain/source electrodes are Au thin films; fully transparent TFTs onglass/ITO substrates (right): the dielectric is a 16.5 nm self-assembledSAS dielectric, and drain/source electrodes are high-conductivity In₂O₃thin films. (D) Molecular structure of a representative nanoscopic SASdielectric and its component constituents.

FIGS. 1E-G. Inorganic-only and inorganic-organic hybrid TFTs fabricatedusing In₂O₃ thin films as the n-channel semiconductor and CPB dielectricas the gate insulator: E) TFTs on doped Si gate substrates with Audrain/source electrodes; F) flexible TFTs on PET/ITO substrates withhigh-conductivity In₂O₃ drain/source electrodes. G) Molecular structureof a representative crosslinked polymer blend (CPB) dielectric.

FIGS. 2A-D. X-ray diffraction θ-2θ scans of In₂O₃ thin films from threeTFT structures: (A) p⁺-Si/SiO₂/In₂O₃; (B) n⁺-Si/(SASnanodielectric)/In₂O₃; (C) n⁺-Si/(polymer dielectric)/In₂O₃; and (D)Hall-effect mobility versus carrier density.

FIGS. 3A-C. AFM images of In₂O₃ thin films from three TFT structures:(A) p⁺-Si/SiO₂/In₂O₃; (B) n⁺-Si/(SAS nanodielectric)/In₂O₃; (C)n⁺-Si/polymer dielectric/In₂O₃.

FIGS. 4A-B. Optical characteristics of 120 nm as-deposited In₂O₃ thinfilms on clean Eagle 2000 glass: (A) Optical transmittance spectrum; (B)derivation of the optical band gap.

FIG. 5. X-ray reflectivity of n⁺-Si/(SAS nanodielectric)/In₂O₃ filmsusing an asymmetric Ge(111) compressor.

FIG. 6. Secondary ion mass spectrum (SIMS) depth profile ofinorganic-organic hybrid TFTs: n⁺-Si/(SAS nanodielectric)/In₂O₃.

FIG. 7. SIMS spectra of inorganic-organic hybrid TFTs: n⁺-Si/(SASnanodielectric)/In₂O₃. The labeling of each spectrum corresponds to thenumbers in FIG. 6. Note that the peak of 69 is from Ga⁺ ion source.

FIGS. 8A-G. Field-effect device characteristics of inorganic-only TFTson p+ Si substrates and inorganic-organic hybrid TFTs on n+ Sisubstrates and Corning 1737F glass substrates; (A-B), Field-effectdevice characteristics of inorganic-only TFTs on p+ Si substrates:current-voltage output characteristics as a function of gate voltage(A); TFT transfer characteristics of current versus gate voltage (B)(thin-film In₂O₃ as the semiconductor (100 μm (L)×5 mm(W)) and 300 nmSiO₂ as the gate dielectric, with Au drain/source electrodes). C,D,Field-effect device characteristics of inorganic-organic hybrid TFTs onn+ Si substrates: current-voltage output characteristics as a functionof gate voltage (C); TFT transfer characteristics of current versus gatevoltage (D) (thin-film In₂O₃ as the semiconductor (50 μm (L)×5 mm(W))and a 16.5 nm SAS dielectric with Au drain/source electrodes). E-F,Field-effect device characteristics of inorganic-organic hybrid TFTs onCorning 1737F glass substrates: current-voltage output characteristicsas a function of gate voltage (E); TFT transfer characteristics ofcurrent versus gate voltage (F) (thin-film In₂O₃ as the semiconductor(100 μm (L)×5 mm(W)) and a 16.5 nm SAS dielectric with Au drain/sourceelectrodes). (G), Comparison of TFT transfer current as a function ofaccumulated charge-carrier density: p+ Si/SiO₂/In₂O₃/Au (left) and n+Si/SAS/In₂O₃/Au (right). Note that inspection of the plots revealspossible contact resistance effects, indicating that performance mightbe enhanced by contact optimization.

FIGS. 9A-C. Typical field-effect device characteristics of fullytransparent inorganic-organic hybrid TFTs on Corning 1737F glasssubstrates. (A) Current-voltage output characteristics as a function ofgate voltage; (B) TFT transfer characteristics of current versus gatevoltage (thin-film In₂O₃ as the semiconductor (100 μm (L)×5 mm(W)) and a16.5 nm SAS gate dielectric on glass/ITO substrates withhigh-conductivity In₂O₃ drain/source electrodes); and (C) Transmissionoptical spectrum of an array of 70 transparent inorganic-organic hybridTFTs (glass/ITO/SAS/In₂O₃/In₂O₃ drain and source electrodes) takenthrough the In₂O₃ drain/source region; transmission optical spectra ofglass/ITO/SAS and glass/ITO/SAS/In₂O₃ structures are also shown forcomparison.

FIG. 10. Secondary ion mass spectrometric (SIMS) depth profile analysisof an n⁺-Si/SAS/In₂O₃ structure.

FIGS. 11A-D. Typical field-effect characteristics of inorganic-organichybrid TFTs using 60 nm thin-film In₂O₃ as the channel layer and a 70 nmCPB as the gate insulator on n⁺-Si substrates with Au drain/sourceelectrodes: (A) transfer current-voltage characteristics; (B) outputcharacteristics as a function of gate voltage; typical field-effectcharacteristics of flexible inorganic-organic hybrid TFTs using 60 nmthin-film In₂O₃ as the channel layer and a 165 nm CPB as the gateinsulator on PET/ITO substrates with Au drain/source electrodes; (C)transfer current-voltage characteristics; and (D) output characteristicsas a function of gate voltage.

FIGS. 12A-C. Typical field-effect characteristics of fully transparentand flexible inorganic-organic hybrid TFTs using 60 nm thin-film In₂O₃as the channel layer and a 165 nm CPB as the gate insulator on PET/ITOsubstrates with high-conductivity In₂O₃ drain/source electrodes: (A)transfer current-voltage characteristics; (B) output characteristics asa function of gate voltage; and (C) Transmission optical spectrum of anarray of 30 transparent inorganic-organic hybrid TFTs(PET/ITO/SAS/In₂O₃/In₂O₃ drain and source electrodes) taken through theIn₂O₃ drain/source region; transmission optical spectrum of blank PETand normalized transmission optical spectrum of PET/ITO/SAS/In₂O₃(referenced to blank PET) are also shown for comparison.

FIGS. 13A-B. Typical field-effect device characteristics of pureinorganic TFTs using a thin-film ZnO semiconductor and SiO₂ dielectricon p⁺-Si substrates: (A) current-voltage output characteristics as afunction of gate voltage; and (B) TFT transfer characteristics ofcurrent vs. gate voltage.

FIGS. 14A-B. Typical field-effect device characteristics ofinorganic-organic hybrid TFTs using thin-film ZnO as the semiconductorand cross-linked polymer dielectrics on n⁺-Si substrates: (A)current-voltage output characteristics as a function of gate voltage;and (B) TFT transfer characteristics of current vs. gate voltage.

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide various semiconductor and dielectric components and/ortransistor devices and related methods for their production and/orassembly, thereby overcoming various deficiencies and shortcomings ofthe prior art, including those outlined above. It will be understood bythose skilled in the art that one or more aspects of this invention canmeet certain objectives, while one or more other aspects can meetcertain other objectives. Each objective may not apply equally, in allits respects, to every aspect of this invention. As such, the followingobjects can be viewed in the alternative with respect to any one aspectof this invention.

It can be an object of the present invention to provide materialcomponents, structures and/or device configurations meeting theaforementioned requirements while, in addition, fully realizing thebenefits available from TFT technologies.

It can be an object of the present invention, alone or in conjunctionwith the preceding objective, to provide an inorganic semiconductorcomponent, as can comprise but is not limited to a metal oxide, withfavorable performance properties, including crystallinity andfield-effect mobilities, as can be available through fabrication at ornear room temperatures or at temperatures non-deleterious totemperature-sensitive substrates.

It can be an object of this invention to provide a wide range of organicor inorganic dielectric materials compatible with a variety ofsubstrates, including organic and/or flexible substrates, compatiblewith a variety of inorganic semiconductors (including both n- andp-type), and enable efficient operation of such semiconductorcomponents.

It can be another object of the present invention, alone or inconjunction with one or more of the preceding objectives, to provide adielectric component with favorable performance properties including butnot limited to capacitance and thermal stability.

It can be another object of the present invention, alone or inconjunction with one or more of the preceding objectives to providevarious compatible combinations of such components through thefabrication of a range of transistor configurations and related devicestructures.

Other objects, features, benefits and advantages of the presentinvention will be apparent from this summary and the followingdescriptions of certain embodiments, and will be readily apparent tothose skilled in the art having knowledge of thin film transistordevices, transistor components and related assembly/fabricationtechniques. Such objects, features, benefits and advantages will beapparent from the above as taken into conjunction with the accompanyingexamples, data, figures and all reasonable inferences to be drawntherefrom, alone or with consideration of the references incorporatedherein.

In part, the present invention can be directed to a transistor device,such a device comprising a substrate, a dielectric component on orcoupled to the substrate, and an inorganic semiconductor component.Available substrate materials are known in the art and include, withoutlimitation, various flexible plastics. The inorganic semiconductorcomponent can be coupled to the dielectric component, and the twocomponents together can form a thin film composition. As used herein,“coupled” can mean the simple physical adherence of two materialswithout forming any chemical bonds (e.g. by adsorption), as well as theformation of chemical bonds (e.g., ionic or covalent bonds) between twoor more chemical moieties, atoms, or molecules.

The present semiconductor components can comprise a metal oxide such asbut not limited to indium oxide and zinc oxide, as well as otherinorganic materials of the sort discussed herein. For instance, suchcomponents can comprise other available Group 12 metals, Group 13metals, Group 14 metals, and Group 15 metals, particularly oxidesthereof, as well as nitrides, phosphides, and arsenides thereof, aswould be understood by those skilled in the art. In some embodiments,the oxides can include two or more metals. In certain embodiments, thesemiconductor component can include two or more different types of metaloxides. Alternatively, from a structural-functional perspectivedescribed more fully below, such semiconductor components can comprise ametal oxide providing advantageous field-effect mobilities, suchmobilities as can be approached through improved component crystallinityand interfacial and related morphological considerations of the typedescribed herein.

Alternatively, with respect to a broader aspect of this invention, aninorganic semiconductor component can comprise one or moresemiconducting metal or metalloid chalcogenides (e.g., sulfides,selenides, tellurides), pnictinides (e.g., gallium, indium, thallium),carbides, halides and the like. The semiconductor component can bedeposited/applied at relatively low temperatures (e.g., at roomtemperature) over relatively large surface areas and combined with ahigh-capacitance organic gate dielectric, of the sort described herein,which can also be deposited at relatively low temperatures overrelatively large surface areas. Such inorganic components are wideranging (e.g., single crystal Si, amorphous Si, GaAs, and variousoxides, nitrides, phosphides and the like) and limited only by theirsemiconductor function, field-effect mobilities and/or use inconjunction with organic or inorganic dielectric components, such useand dielectric components as described more fully below. Regardless,such a semiconductor can comprise a thin film, wire, nanowire, nanotubeor nanoparticle configuration, or as otherwise could function in thecontext of a particular device structure—such configurations as would beunderstood by those skilled in the art.

A dielectric component of this invention can be selected from variousmaterials providing favorable capacitance and/or insulating properties.In some embodiments, the dielectric component can include at least oneof a multi-layered organic assembly, an organic polymeric composition,and silicon dioxide. Such dielectric components, as would also beunderstood by those in the art made aware of this invention,

In certain non-limiting embodiments, the dielectric component caninclude a multi-layered organic assembly/composition having periodicallyalternating layers of different materials. These alternating layers caninclude one or more layers that include a π-polarizable moiety (“achromophore layer”), and one or more layers that include a silyl orsiloxane moiety (“an organic layer”). At least some of the alternatinglayers can be coupled by a coupling or capping layer that includes asiloxane matrix.

The π-polarizable moiety can include conjugated π-electrons. In someembodiments, the π-polarizable moiety can include at least one of adipole moment, an electron releasing moiety, an electron withdrawingmoiety, a combination of such moieties, a zwitterion and a net charge.Without limitation, such a component can comprise a non-linear optical(NLO) chromophore. In some embodiments, the chromophore can include aπ-conjugated system, which can include a system of atoms covalentlybonded with alternating single and multiple (e.g. double) bonds (e.g.,C═C—C═C—C and C═C—N═N—C). The π-conjugated system can includeheteroatoms such as, but not limited to, nitrogen (N), oxygen (O), andsulfur (S). In some embodiments, the π-conjugated system can include oneor more aromatic rings (aryl or heteroaryl) linked by conjugatedhydrocarbon chains. In certain embodiments, the aromatic rings can belinked by conjugated chains that include heteroatoms (e.g., azo groups[—N═N—]). For example, the π-polarizable moiety can be a chromophorethat includes a stilbazolium moiety. The identity of such compounds arelimited only by their electronic/structural features and resultingpolarizability in the context of a particular use or application, asillustrated by various representative embodiments described herein.

The organic layers can include a bis(silylated)alkyl moiety (e.g.,ranging from about C₁ to about C₂₀). In particular embodiments, theorganic layers can be coupled to the chromophore layers directly or viaa coupling or capping layer that includes a siloxane matrix. Thecoupling can be performed via a condensation reaction or chemisorptionusing known silyl chemistry. For example, precursors of the silyl moietyand the siloxane moiety can include hydrolyzable groups such as, but notlimited to, halo groups, amino groups (e.g., dialkylamino groups), andalkoxy groups. Examples of such precursors can include, but are notlimited to, Cl₃Si(CH₂)_(n)SiCl₃, (CH₃O)₃Si(CH₂)_(n)Si(OCH₃)₃, and(Me₂N)₃Si(CH₂)_(n)Si(NMe₂)₃, where n can be an integer in the range of1-10 (i.e., n can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10). As discussedmore fully herein, such groups are hydrolyzable to a degree sufficientfor substrate sorption or condensation or intermolecular crosslinkingvia siloxane bond formation under the processing or fabricationconditions employed. Similarly, the π-polarizable moiety can bederivatized to include similar silyl hydrolyzable groups, to allow bondformation with the siloxane capping layer and/or the organic layer. Inparticular embodiments, the organic layers and the chromophore layerscan be individually self-assembled monolayers that include the silyl orsiloxane moiety, or the n-polarizable moiety.

In some embodiments, the dielectric component can comprise at least oneorganic dipolar layer comprising a compound comprising a π-polarizablemoiety covalently bonded to or cross-linked with a siloxane bondsequence. In certain embodiments, such a dielectric component cancomprise a hydrocarbon layer coupled with silicon-oxygen bonds to such adipolar layer. In certain other embodiments, such a dielectric componentcan further comprise at least one siloxane capping layer coupled to sucha dipolar layer, with silicon-oxygen bonds. Regarding such embodiments,a siloxane capping layer can be positioned between a dipolar layer and ahydrocarbon layer, coupled to each with silicon-oxygen bonds. Suchcomponents are described more fully in co-pending application Ser. No.11/181,132, filed Jul. 14, 2005, the entirety of which is incorporatedherein by reference.

Such a silicon-oxygen bonding sequence can be the condensation productof a hydrolyzable silicon moiety (e.g., without limitation, ahalogenated, alkoxylated and/or carboxylated silyl moiety) and ahydroxyl functionality. As understood in the art and explained morefully in one or more of the references incorporated herein, such abonding sequence can derive from use of starting material compounds forthe respective dielectric layers, such compounds substituted with one ormore hydrolyzable silicon moieties, hydrolysis of such a moiety underself-assembly conditions, and condensation with a subsequent layerstarting material or precursor compound.

Precursor compounds which can be incorporated into such layers caninclude, for instance, bis-trichlorosilyloctane, octachlorotrisiloxaneand4-[[(4-(N,N-bis((hydroxy)ethyl)amino]-phenyl]azo]-1-(4-trichlorosilyl)benzyl-pyridiniumiodide, such compounds condensed one with another, with correspondinglayers assembled to provide dielectric components in accordance withthis invention.

While several of the aforementioned dielectric component compounds,layers and moieties are illustrated in the aforementioned incorporatedreference, various other component compounds and associated moieties arecontemplated within the scope of this invention, as would be understoodby those skilled in the art made aware thereof. For instance, withoutlimitation, various other π-polarizable component compounds andassociated moieties are described in U.S. Pat. No. 6,855,274, inparticular the NLO structures of FIGS. 1-2, 11, 13 and 15 thereof, U.S.Pat. No. 6,549,685, in particular FIGS. 2-3 thereof, and U.S. Pat. No.5,156,918, in particular the structures of FIGS. 4-5 thereof, each withreference to the corresponding specification regarding alternateembodiments synthesis and characterization, each of which isincorporated herein by reference in its entirety. Further, as would beunderstood by those skilled in the art, various other non-linear opticalchromophore compounds are described in “Supramolecular Approaches toSecond-Order Nonlinear Optical Materials. Self-Assembly andMicrostructural Characterization of Intrinsically Acentric[(Aminophenyl)azo]pyridinium Superlattices”, Journal of AmericanChemical Society, 1996, 118, 8034-8042, which is hereby incorporated byreference in its entirety. Such layer component compounds can be used,as described herein, with a variety of difunctionalized hydrocarbonlayer and/or siloxane capping layer component compounds, such compoundswithout limitation as to hydrocarbon length or degree offunctionalization capable of condensation with a suitable substrateand/or various other dielectric layers or components in accordance withthis invention.

In certain embodiments, the multi-layered dielectric component can alsoinclude one or more layers that include an inorganic moiety (“aninorganic layer”). The inorganic layers can periodically alternate amongthe organic layers and the chromophore layers, and can include one ormore main group and/or transition metals. For example, the metal(s) canbe selected from a Group 3 metal, a Group 4 metal, a Group 5 metal, anda Group 13 metal. In particular embodiments, the main group metal(s) canbe selected from a Group 13 metal such as, but not limited to, gallium(Ga), indium (In), and thallium (Tl) etc., and the transition metal canbe selected from a Group 3 metal such as, but not limited to, ittrium(Y), a Group 4 metal such as, but not limited to, titanium (Ti),zirconium (Zr), and hafnium (Hf), and a Group 5 metal, such as but notlimited to, tantalum (Ta).

In some embodiments, the dielectric component can comprise a dielectricpolymeric component and optionally a silylated component comprising amoiety, e.g., an alkyl group or a haloalkyl group, linking two or moresilyl groups having hydrolyzable moieties. Various other linkingmoieties will be recognized in the art, limited only by structure orfunctionality precluding intermolecular siloxane bond and matrixformation. The range of the hydrolyzable silyl groups will be known bythose skilled in the art made aware of this invention, and include butare not limited to groups such as trialkoxysilyl, trihalosilyl,dialkoxyhalosilyl, dialkylhalosilyl, dihaloalkylsilyl anddihaloalkoxysilyl. Such polymeric compositions are described more fullyin co-pending application Ser. No. 60/638,862, filed Dec. 23, 2004, theentirety of which is incorporated herein by reference.

In certain non-limiting embodiments, a bis(silylated) component cancomprise an alkyl moiety ranging from about C₁ to about C₂₀, linking twotrihalosilyl groups, two trialkoxysilyl groups or a combination thereof.As discussed more fully herein, such groups are hydrolyzable to a degreesufficient for substrate sorption or condensation or intermolecularcrosslinking via siloxane bond formation under the processing orfabrication conditions employed. Regardless, the polymeric component ofsuch compositions can be selected from a range of such dielectricpolymers otherwise used in the art as separate gate insulator materialsor layers in OTFT fabrication. For purpose of example only, dielectricpolymers can include poly(vinylphenol), polystyrene and copolymersthereof. In some embodiments, such polymeric compositions can becrosslinked. Such compositions of this invention are limited only by theavailability of suitable silylated components and polymeric dielectriccomponents, the mixture or miscibility thereof one with another fordevice fabrication, and the resulting polymer-incorporatedsiloxane-bonded matrix/network and corresponding dielectric/insulatorfunction.

From a device perspective, in certain non-limiting embodiments, thisinvention can comprise various high-performance inorganic-organic hybridTFTs fabricated, for example, with semiconducting thin-films as then-channel or p-channel material and a range of thin organic dielectrics.Alternatively, in other embodiments, TFTs can also comprise inorganicdielectrics such as but not limited to SiO₂. As shown below,representative In₂O₃ films can be deposited at room temperature byion-assisted deposition (IAD) sputtering, and all TFTs can be fabricatedat room/near-room temperatures. Semiconducting In₂O₃ films, organicdielectrics, and TFT device structures were characterized in detail. Itis found that inorganic semiconductor components with sufficientmicrostructural crystallinity exhibit n-type or p-type field-effectbehavior, and thin organic (or, e.g. SiO₂) dielectric components withsufficient insulating properties enable ultra-low-voltage TFT operation.Such hybrid TFTs can show exceptionally large field-effect mobilitiesof >100 cm²/V s at low operating voltages (1˜2 V). Furthermore, such afabrication approach is shown to be applicable to transparent flexibleplastic substrates, as well as other temperature-sensitive substratematerials.

As such, one aspect of the invention therefore is directed to a TFTdevice that includes a substrate (including a substrate-gate materialsuch as, but not limited to, doped-silicon wafer, tin-doped indium oxideon glass, tin-doped indium oxide on mylar film, and aluminum onpolyethylene terephthalate), a dielectric material as described hereindeposited on the substrate/substrate-gate, a semiconductor materialdeposited on the dielectric material, and source-drain contacts. In someembodiments, the TFT can be a transparent TFT including one or more ofthe followings: a transparent or substantially transparent substrate, atransparent or substantially transparent gate conductor, a transparentor substantially transparent inorganic semiconductor component, atransparent or substantially transparent dielectric component, andtransparent or substantially transparent source and drain contacts. Asused herein, “transparent” refers to having at least a 90% transmittancein the visible region of the spectrum, and “substantially transparent”refers to having at least 80% transmittance in the visible region of thespectrum.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS

Throughout the description, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including, or comprising specific process steps, itis contemplated that the compositions also consist essentially of, orconsist of, the recited components, and that the processes also consistessentially of, or consist of, the recited processing steps.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components and can be selected from a groupconsisting of two or more of the recited elements or components.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. In addition, where the use of theterm “about” is before a quantitative value, the invention also includesthe specific quantitative value itself, unless specifically statedotherwise.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the method remains operable.Moreover, two or more steps or actions can be conducted simultaneously.

Various embodiments of this invention can be considered in light of thefollowing: dielectric and semiconductor TFT components, and relatedparameters for evaluating TFT performance, such as the field-effectmobility (μ) and the drain-source current on/off ratio (I_(on):I_(off)).High μ values lead to large drain current, fast charged/dischargedcapacitive loads, high operating speeds, and thus enable wide range ofapplications. I_(on) and I_(off) are the gate-controlled drain-sourcecurrent (I_(DS)). I_(DS) in the saturation region is expressed byEquation 1, where W and L are the channel width and length,respectively, V_(T) is the threshold voltage, and C_(i) is thedielectric capacitance per unit area. The threshold voltage V_(T) isdefined as the V_(G) at which a device switches from the off state tothe on state, and vice versa. Ideally, V_(T) should be minimal (e.g.,0.0V) to minimize power consumption. C_(i) is expressed by Equation 2,where k is the dielectric constant, ∈_(o) is the vacuum permittivity,and d is the dielectric thickness.

$\begin{matrix}{I_{DS} = {\frac{{WC}_{i}\mu}{2L}\left( {V_{G} - V_{T}} \right)^{2}}} & (1) \\{C_{i} = \frac{k\; ɛ_{0}}{d}} & (2)\end{matrix}$Note that for a certain device geometry, a given I_(DS) can be achievedat lower operating bias by increasing μ in the semiconductor orincreasing C_(i) in the dielectric. However, previous efforts have onlyfocused on one or the other of these parameters. To simultaneouslyachieve a large μ and a large C_(i), suitable semiconductors anddielectrics are needed, with sufficient compatibility therebetween.

Metal oxides are a class of semiconductor or component materials fortransparent flexible TFTs. The attraction of metal oxides includes highmobility, wide band gap, broad visible window, controllable electricalproperties, and room-temperature growth. So far, metal-oxide-based TFTshave not been optimized, and the majority of metal-oxide-based TFTsdemand either high growth temperatures or post-annealing treatment(usually >500° C.) in order to improve the crystallinity and thereforethe field-effect mobility. However, high-temperature processing preventsapplications in most flexible (polymer-based) electronics. Previousmetal-oxide-based TFTs fabricated at room temperature exhibit poorperformance, in particular low field-effect mobilities, lowI_(on):I_(off) ratios and large operating voltages, any of which tend topreclude most practical applications. Moreover, conventional growthtechniques tend to be incompatible with practical large-area/scaledeposition and device fabrication.

In₂O₃ is a promising n-type material having a wide band gap (3.6˜3.75eV), high mobility as single crystals (160 cm²/V s), considerabletransparency in the visible region (>90%), but has not been usedpreviously to fabricate TFT devices. See, respectively, Radha Krishna,B.; Subramanyam, T. K.; Srinivasulu Naidu, B.; Uthanna, S. Opt. Mater.2000, 15, 217; Weiher R. L.; Ley, R. P. J. Appl. Phys. 1966, 37, 299;Weiher, R. L. J. Appl. Phys. 1962, 33, 2834; Wang, L.; Yang, Y.; Marks,T. J.; Liu, Z.; Ho, S.-T. Appl. Phys. Lett. In press. On the other hand,thin organic dielectrics (e.g., self-assembled multilayer andcross-linked polymer nanodielectrics) have been proved to remarkablyeffective in enhancing the response characteristics of organic TFTsproperties, while exhibiting good transparency in visible region. Asdemonstrated below, the efficient incorporation of inorganic In₂O₃semiconductors and thin organic dielectrics—both at room temperaturesusing scalable growth processes—can provide transparent flexible TFTswith high mobility, good optical transparency, and low-voltageoperation.

Representative top-contact TFT device structures are shown in FIGS.1A-B. In₂O₃ thin films were deposited on inorganic dielectrics (SiO₂)and organic dielectrics by room-temperature IAD sputtering, arepresentative deposition technique. Organic dielectrics were fabricatedon n⁺-doped Si (100) wafers and transparent flexible ITO/PET substrates(as the back gate; where ITO denotes tin-doped indium oxide) via eithersolution phase spin coating or layer-by-layer self-assembled techniques.Finally, conducting source and drain electrodes (e.g., from Au, ITO, andother suitable materials known in the art) were patterned through shadowmasks, using techniques known in the art.

The structures and components of other hybrid TFTs are illustrated inFIGS. 1C-D, with all components deposited at room temperature. Forcomparison, In₂O₃ TFTs were also fabricated in combination with aconventional 300 nm SiO₂ dielectric layer. The semiconducting In₂O₃ thinfilms, thin nanoscopic dielectrics, TFT device structures and electricalproperties were characterized as described below. Such hybrid TFTs haveexceptionally large field-effect mobilities of 140 cm2 V⁻¹ s⁻¹ at lowoperating voltages (˜1V) with good I_(on)/I_(off) ratios. Furthermore,this fabrication approach is applicable to glass substrates to realize“invisible” TFTs. Different FET device structures can be fabricatedaccording to the teachings herein, including bottom-contact andbottom-gate structures, top-gate top-contacts structures, top-gatebottom-contacts structures, and bottom-gate top-contacts structures.

As understood in the art, IAD applies two ion beams to simultaneouslyeffect film deposition, oxidation, and crystallization, leading tosmooth, dense, coherent films at room temperature. Meanwhile, theassisted ion beam realizes a pre- and in-situ cleaning/activationprocess, producing fresh surfaces, creating strong interfacial adhesion,and achieving full oxidation state/stoichiometry. Thus, IAD is capableof growing high-quality thin films on organic/plastic substrates.Another attraction of IAD is that the electrical properties of resultingfilms can be finely/readily engineered via control of the growth systemoxygen partial pressure and ion beam power during the film growthprocess. For instance, the room-temperature Hall mobilities of theIAD-derived In₂O₃ films (FIG. 2C) are substantial and nearly constant ascarrier concentration is varied over the broad range of 10¹⁷-10²⁰ cm⁻³.For useful semiconduction (low I_(off)), In₂O₃ thin films aredeliberately grown to be highly resistive, indicating that the carrierconcentration can be efficiently suppressed by IAD. Under these growthconditions, the conventional Hall effect mobility is immeasurable byconventional techniques because of the low carrier density. Theconductivity of In₂O₃ thin films is estimated to be ˜10⁻⁴-10⁻⁵ S/cm, andthe carrier concentration is ˜10¹³˜10¹⁴ cm⁻³, as derived from the fieldeffect mobility.

X-ray diffraction (XRD) θ-2θ scans of In₂O₃-based TFTs (without Ausource and drain electrodes) reveal that the In₂O₃ films possessconsiderable crystallinity (i.e., cubic bixebyte structure) when grownon SiO₂ (FIG. 2A), and that films on p⁺-Si/(thin organic (e.g., SAS)dielectric) exhibit even stronger textured crystallinity judging fromthe peak widths of the dominant XRD peaks (FIGS. 2B and 2C).Crystallinity strongly/positively influences the field effect mobilityand TFT device performance, and thus is desired in the semiconductingchannel materials. With this microstructural crystallinity, an In₂O₃semiconductor thin film can be used effectively as a channel material incertain TFT embodiments.

The surface morphologies and grain sizes of IAD-derived In₂O₃ thin filmswere examined by contact-mode AFM and images are shown in FIGS. 3A-D.The In₂O₃ thin films grown on the different dielectric substrates arecompact, dense, uniform, and smooth, and exhibit low RMS roughnesses,e.g., RMS=0.7-0.8 nm on Si/SiO₂, 1.6-1.8 nm on the Si/organic (SAS)dielectric, 2.7-3.1 nm on Si/cross-linked polymer dielectric, and1.9-2.1 on glass/ITO/SAS. Low roughness can be attributed to: (1) thesmooth underlying dielectric (whether organic or inorganic), which isfurther supported by AFM; and (2) the intrinsic efficacy of the IADgrowth technique to deposit smooth films.

All the present as-grown In₂O₃ films are colorless and highly opticallytransparent, and the films on the glass show an average transparency of˜90% in visible region (FIG. 4A). The direct optical band gap wasinvestigated and estimated from the optical transmittance spectrum byextrapolating the linear part of the plot of (αhυ)² versus hυ to α=0.Band gap data shows a typical value of 3.65 eV for the IAD-derived In₂O₃films (FIG. 4B). Transmittance and band gap results suggest that In₂O₃thin films are ideal re-channel materials for transparent TFTfabrication. The room-temperature growth process expands the applicationof In₂O₃ semiconductors to transparent flexible substrates.

Organic dielectrics were fabricated (e.g., self-assembled orspin-coated) by solution-phase-based growth techniques, leading to goodsmoothness, strong adhesion, good thermal stability, pin-hole-free andremarkable electrical insulating characteristics. For instance,self-assembled superlattice (SAS) nanodielectrics, of the type describedmore fully in several of the incorporated references, exhibit a largecapacitance of 180 nF/cm², an effective dielectric constant of 4.7,leakage current as low as 10⁻¹³ pA, and breakdown fields as high as 4MV/cm, as determined from the capacitance measurement. As a result, thinorganic dielectrics promise TFT operation at very low gate anddrain-source voltages. Note that such thin organic dielectrics aremechanically/chemically robust, and careful control of the IAD growthprocess ensures that such dielectric materials survive the ion/plasmaexposure during metal oxide deposition.

The multilayer structures and composition of the present hybrid TFTswith the device structure of n⁺-Si/(SAS nanodielectrics)/In₂O₃ wereinvestigated by X-ray reflectivity (XRR) (FIG. 5) and secondary ion massspectroscopy (SIMS) quantitative in-depth analysis (FIGS. 6 and 7). TheXRR fringe pattern persists to large q values (as much as q=0.3),qualitatively demonstrating smooth interfaces of hybrid TFTs. Inaddition, SIMS depth profile results and spectra show these devices haveabrupt In₂O₃-dielectric interfaces, minimal interfacial cross-diffusion,and phase purity. Clean interfaces in principle minimize electron trapsand hysteresis, and should thereby enhance μ_(FE). Generally, weakadhesion between inorganic and organic interfaces is a significantfactor degrading organic field-effect transistor performance andstability. For the present devices, the conventional ‘Scotch tape’adhesion test reveals no detectable change in multilayer thickness,optical microscopic images or optical transparency before and after thetest, indicating that IAD-grown In₂O₃ films on the organic dielectricsexhibit strong interfacial adhesion.

In₂O₃-based TFTs were first characterized on p⁺-Si substrates having aconventional SiO₂ dielectric, next on n⁺-Si substrates with the SAS andCPB dielectrics (FIGS. 8A-E), and then on glass/indium tin oxide (ITO)substrates with the SAS dielectric (FIGS. 8E-F and 5). TFT deviceresponse parameters are summarized in Table 1. The In₂O₃ devices usingSiO₂ gate dielectrics show reasonable field-effect responses (μ_(FE)=10cm² V⁻¹s⁻¹; I_(on)/I_(off)=10⁵) with operating voltages in the 100 Vrange (FIGS. 8A-B, Table 1). In contrast, inorganic-organic hybrid TFTsfabricated on n⁺-Si/SAS substrates exhibit excellent I-V characteristics(FIGS. 8C-D, Table 1) with classical/crisp pinch-off linear curves andsaturation lines at very low operating voltages. Low operating voltages(˜1V) are essential for mobile electronics powered by simple householdbatteries.

TABLE 1 Materials and device parameters for TFTs fabricated from In₂O₃thin films + SiO₂ or nanoscopic organic SAS*/CPB^(†) dielectrics on Siand glass/ITO gates using Au or In₂O₃ drain and source electrodes. In₂O₃thickness Dielectric/thickness D & S Gate (nm) (nm)/C_(i) (nF cm⁻²)electrodes 1. p⁺-Si 120 SiO₂/300/10 Au 2. n⁺-Si 60 SAS*/16.5/180 Au 3.n⁺-Si 60 CPB^(†)/20/250 Au 4. Glass/ITO 60 SAS*/16.5/180 Au 5. Glass/ITO60 SAS*/16.5/180 In₂O₃ μ_(FE) μ_(GB) ^(‡) I_(on)/ V_(t) N_(t) ^(§) S (Vper (cm²V⁻¹s⁻¹) (cm²V⁻¹s⁻¹) I_(off) (V) (cm⁻²) decade) 1. 10 24 10⁵ 231.73 × 10¹² 5.6 2. 140 178 10⁵ 0.33 2.33 × 10¹¹ 0.15 3. 80 94 10³ 0 2.79× 10¹¹ 0.41 4. 140 181 10⁵ 0.17 2.44 × 10¹¹ 0.08 5. 120 154 10⁵ 0.192.51 × 10¹¹ 0.09 *SAS = Self-assembled superlattice dielectric. ^(†)CPB= Poly-4-vinylphenol + 1,6-bis(trichlorosilyl)hexane dielectric.^(‡)μ_(GB) = Grain-boundary mobility. ^(§)N_(t) = Trap density.

Analysis of the n⁺-Si/SAS/In₂O₃ device electrical response reveals largesaturation-regime field-effect mobilities of ˜140 cm² V⁻¹s⁻¹,encouraging for high-speed applications. Such mobilities are ˜10×greater than previously reported for metal oxide TFTs fabricated at roomtemperature, and are attributed to the following: (1) substantialcrystallinity of the IAD-derived In₂O₃ semiconductor, verified by theXRD results and grain-boundary trapping model analysis (vide infra), andthe resulting suppressed neutral impurity scattering, (2) very smallvalues of the interfacial trap density (N_(t)˜10¹¹ cm⁻², Table 1), (3)minimal ionized-impurity scattering (the carrier concentration is˜10¹³-10¹⁴ cm⁻³) (4) strong adhesion and smooth, abruptsemiconductor/dielectric interfaces to minimize electron traps and (5)advantageous characteristics of the high-capacitance organic nanoscopicdielectrics. The threshold voltage V_(T) of the present devices is closeto 0.0 V, with nearly hysteresis-free response and minimal trappedcharge. That electrons are generated by a positive gate bias V_(G),verifies that In₂O₃ shows n-channel behavior. Furthermore,I_(on)/I_(off) ratios of ˜10⁵ are achieved, and the maximum drain-sourcecurrent reaches the mA level, sufficient for most portable circuitapplications. Small subthreshold gate voltage swings of 150 mV perdecade (Table 1) are achieved at the maximum slope for devicesfabricated with the SAS dielectric (FIG. 8D).

To correct for the differences in the different dielectric layers (forexample capacitance, thickness and applied bias), the TFT transfercurrent characteristics are plotted versus accumulated charge carrierdensity (FIG. 8G). It can be seen that the n⁺-Si/SAS/In₂O₃/Au hybriddevices switch on at much lower accumulated charge carrier densitiesthan the inorganic-only p⁺-Si/SiO2/In₂O₃/Au devices, revealing thatelectron mobilities and charge-injection efficiency between the In₂O₃channel and drain/source electrodes are markedly greater in the hybridTFT case.

To further investigate the generality and applicability of the presentinorganic-organic hybrid TFT strategy, In₂O₃ TFTs were next fabricatedon n⁺-Si/(20 nm CPB dielectric) with Au source and drain electrodes.These n⁺-Si/CPB/In₂O₃ devices also have good field-effect electricalresponse with field-effect mobilities of ˜80 cm² V⁻¹ s⁻¹, I_(on)/I_(off)ratios of 10³, and low V_(T)˜0.0 V. Next, using the same fabricationtechniques, hybrid TFTs were grown at room temperature onglass/IAD-ITO/SAS gate structures with Au drain/source electrodes (FIGS.8E-F, Table 1). Such TFTs also have excellent field-effect I-Vcharacteristics with large field-effect mobilities of 140 cm² V⁻¹ s⁻¹,high I_(on)/I_(off) ratio of 105, near 1.0V operation withnon-hysteretic characteristics, and small gate voltage swings of only 80mV per decade.

Using high-conductivity IAD-derived In₂O₃ as drain and source electrodesaffords completely transparent TFTs (FIG. 9), having field-effectmobilities of 120 cm² V⁻¹ s⁻¹, large on/off ratios of 10⁵, ˜1.0 Voperation, essentially hysteresis free characteristics and very smallsub-threshold gate voltage swings of 90 mV per decade. Note that suchsmall gate voltage swings (benefiting from both the high-quality In₂O₃semiconductor and the high-capacitance organic nanoscopic dielectric)are only slightly larger than the theoretical limit for Si-based TFTs(˜60 mV per decade). Estimation of the grain-boundary mobilities μ_(GB),using a grain-boundary trapping model, provides important information onintrinsic carrier transport characteristics within a singlesemiconductor grain (Table 1). It can be seen that μ_(GB) trends are ingood agreement with the crystallinity results provided by XRD (FIGS.2A-B) and that the values of μ_(GB) are slightly larger than values ofμ_(FE), suggesting that grain-boundary scattering/trapping in theseIn₂O₃ films may limit μ_(FE). That these hybrid TFTs on glass substratesare colorless and highly transparent is shown by transmission opticalspectra (FIG. 9C). The transmittance of an entire 70-device TFT array(glass/ITO/SAS/In₂O₃/In₂O₃ drain and source electrodes) is >80% in thevisible region, and a colourful pattern beneath the aforementioned TFTarray can easily be seen.

Following a similar approach, transparent flexible hybrid TFTsfabricated with organic CPB gate dielectrics were spin-coated bysolution-phase techniques at near room temperature. By using commercialavailable starting materials, organic CPB dielectric films with greatcohesion and insulating characteristics (robust, smooth, conformal,pin-hole-free, thermally stable) can be deposited on a variety of rigidand flexible substrates. Structures, components, and processes relatingto such hybrid TFTs are shown in FIGS. 1E-G. The semiconducting In₂O₃channel layers, CPB dielectrics, TFT device structures, and electricalproperties, were characterized as described below. Such hybrid TFTsexhibit excellent field-effect characteristics on Si substrates, andthis fabrication approach is applicable to plastic substrates—e.g., PETto realize flexible “invisible” TFTs with good performance.

The microstructures and surface morphologies of an In₂O₃ channel layerin such present hybrid TFTs were characterized by X-ray diffraction(XRD) θ-2θ scans and contact-mode atomic force microscope (AFM),respectively (FIGS. 2C and 3C). Obviously, In₂O₃ films possesses thecharacteristic cubic bixebyte structure with (100)-series orientationand considerable crystallinity/texture even when grown on n⁺-Si/CPB atroom temperature, evidenced by the narrow XRD peak width. AFM imagesshow the In₂O₃ thin films grown on n⁺-Si/CPB to be compact, dense, anduniform, exhibiting relatively small RMS roughnesses of 3.3±0.2 nm onn⁺-Si/CPB substrates and 6.9±0.3 nm on PET/ITO/CPB substrates. Note thatn⁺-Si surface morphologies (RMS roughness: ˜0.1 nm) are much smootherPET surface (RMS roughness: ˜2.2 nm), which to some extent contribute toIn₂O₃ rough surfaces on plastic substrates. The multilayer structuraland compositional characteristics of the present hybrid TFTs wereinvestigated on n⁺-Si/CPB/In₂O₃ structures by secondary ion massspectrometric (SIMS) depth-profiling (FIG. 10). The SIMS results showthat these mutilayer devices have clear channel/dielectric anddielectric/gate interfaces, and minimal interfacial cross-diffusion.Clear channel/dielectric interfaces no double minimize electron trapsand hysteresis effect, and thus lead to large μ_(FE). The conventional“Scotch tape” adhesion test was carried out to inspect the interfacialadhesion at each interface. No detectable change was found in thethickness, optical microscopic images, and optical transparency beforeand after the test, revealing that both CPB/In₂O₃ and gate/CPBinterfaces possess strong interfacial adhesion.

The μ_(FE) in the saturation region is calculated using Equation 1,above; grain-boundary mobilities μ_(GB) and the interfacial trapdensities N_(t) between the semiconductor and dielectric were computedby plotting ln(I_(DS)/V_(G)) at a given drain-source voltage accordingto Equation 3 (the grain boundary trapping model); the subthresholdvoltage swings are obtained at the maximum slope of dV_(G)/d(log I_(DS))based on Equation 4.

$\begin{matrix}{I_{DS} = {\frac{W\;\mu_{GB}V_{DS}C_{i}V_{G}}{L}{\exp\left( \frac{{- q^{3}}N_{t}^{2}t}{8\; ɛ\;{kTC}_{i}V_{G}} \right)}}} & (3) \\{S = \frac{\mathbb{d}V_{G}}{{\mathbb{d}\log}\; I_{DS}}} & (4)\end{matrix}$where q is the electron charge, t is the channel thickness, ∈ is theIn₂O₃ permittivity, k is the Boltzmann constant, and T is the absolutetemperature at room temperature. In₂O₃ ⁻ based TFTs were firstcharacterized on n⁺-Si/CPB substrates, and then on PET/IAD-ITO/CPBsubstrates (FIGS. 1E-F). TFT device response parameters are summarizedin Table 2.

TABLE 2 Component materials and device parameters for TFTs fabricatedfrom In₂O₃ channel layers and organic CPB dielectrics on Si andPET/IAD-ITO gates using Au or In₂O₃ drain and source electrodes.Dielectric In₂O₃ Thickness Thickness (nm)/C_(i) D & S μ_(FE) μ_(GB)V_(T) Gate (nm) (nF/cm²) Electrodes (cm²/V · s) (cm²/V · s)I_(on)/I_(off) (V) N_(t)(cm⁻²) S (V/decade) n⁺-Si 60  70/100 Au 160 17110⁴ 0.21 9.1 × 10¹¹ 0.24 PET/ITO 60 160/30 Au 20 26 10³ 0.12 2.9 × 10¹²2.78 PET/ITO 60 160/30 In₂O₃ 10 22 10² 0.23 3.5 × 10¹² 2.98

The inorganic-organic hybrid TFTs fabricated on n⁺-Si/CPB substratesexhibit remarkable I-V characteristics (FIGS. 11A and 11B) at very lowoperating biases (1-2 V) with classical/crisp pinch-off linear curves inthe linear region and steady/flat lines in the saturation region. Owingto In₂O₃ substantial crystallinity, small interfacial trap densitiesbetween channel layers and dielectric layers (Table 2), and advantagesof the inorganic-organic hybrid TFT structure, large saturation regimefield-effect mobilities of ˜160 cm²/V s were obtained inn⁺-Si/SAS/In₂O₃/Au devices. This large value represents the highestfield-effect mobility in room-temperature TFTs and thus is verypromising for high-speed electronics. The threshold voltage V_(T) of thepresent devices is only 0.21 V with nearly hysteresis-free response,reflecting that In₂O₃ exhibits n-channel materials behavior.Furthermore, I_(on)/I_(off) ratios of ˜10⁴ are achieved, and the maximumdrain-source current reaches the mA level, a sufficient value for mostportable circuit applications. Small sub-threshold gate voltage swing of240 mV/decade (Table 2) is achieved at the maximum slope of thecurrent-voltage output curve (FIG. 11A).

To further explore the versatility and demonstrate the transparency andflexibility of the present inorganic-organic hybrid TFT strategy, In₂O₃TFTs were next fabricated on PET/IAD-ITO/CPB gate structures at roomtemperature with Au drain/source electrodes (FIGS. 11C and 11D). TheseTFTs on plastic PET substrates also exhibit excellent field-effectnon-hysteretic I-V characteristics with large field-effect mobilities of˜20 cm²/V s, I_(on)/I_(off) ratio of 10³, with moderate operatingvoltage and gate voltage swings being 10 V and 2.78 V/decade,respectively.

High-conductivity IAD-derived In₂O₃ drain and source electrodes affordcompletely transparent TFTs (FIGS. 12A-C), having substantialfield-effect mobilities of ˜10 cm²/V s, on/off ratios >10²,sub-threshold gate voltage swings of 2.98 mV/decade, and essentiallyhysteresis-free characteristics. Computed with the grain boundarytrapping model (Equation 3), the grain boundary mobilities μ_(GB) andthe interfacial trap densities N_(t) reveal important information onintrinsic carrier transport and trapping characteristics (Table 2), andexplain the differences of aforementioned TFT performance on twosubstrates (silicon and PET). It can be seen that the grain boundarymobilities on PET are much smaller than that on silicon substrates, andqualitatively agree with the field-effect mobility results, reflectingthat In₂O₃ texture/crystallinity are modest on PET which in turn leadsto somewhat lower field-effect mobilities on PET. On the other hand, thevalue of interfacial trap densities on PET is found to be greater thanthat on silicon substrates, which also partially explains the smallermobilities on PET. Intrinsically, the large interfacial trap density isusually caused by rougher morphology, substrate/film mismatch (e.g.density, thermal expansion, hardness), unoptimized processing procedure,and thereby affects TFT properties, especially the field-effectmobility.

The bending effect on flexible TFTs was investigated by bending theseflexible TFTs into a curve. No significant changes or residual effectswere observed when the bending radius of curvature is as low as 4 cm.That these hybrid TFTs on plastic substrates are colorless and highlytransparent is shown by transmission optical spectra and the photo image(FIG. 12C), where the transmittance of an entire 30-device TFT array(PET/ITO/CPB/In₂O₃/In₂O₃ drain and source electrodes) is ˜80% in thevisible region.

As demonstrated, representative inorganic-organic hybrid TFTs have beenfabricated at room temperature using IAD-derived high-qualitysemiconducting In₂O₃ and organic spin-coatable polymer gate dielectrics.On silicon substrates, the TFTs exhibit field-effect mobilities up to160 cm²/V s, I_(on):I_(off)=10⁴, sub-threshold gate voltage swings of240 mV/decade and function at ˜2.0 V. Integrated with PET substrates andtransparent drain/source electrodes, optical transparency and mechanicalflexibility can be achieved as well as other good field-effectcharacteristics. Furthermore, the present TFT devices are obtained bylarge-scale/large-area fabrication techniques (scalable sputteringinorganic semiconductor and simple spin-coatable organic dielectricprocess) and of all transparent components with good bendability, andthus represent a promising pathway for flexible “invisibles”electronics.

To further demonstrate the generality of the present inorganic-organichybrid approach, ZnO-based TFTs were fabricated on p⁺-Si/SiO₂ andn⁺-Si/(polymer dielectric) at near-room temperature. Clear field-effectresponses were observed for ZnO-based TFTs with complete light- andair-stability. (See, Table 1, example 3 and FIGS. 13-14.) The results onZnO-based TFTs further support that such a hybrid approach can beextended to the use of various other semiconducting metal-oxidematerials.

Such results demonstrate that hybrid integration of an oxidesemiconductor, as illustrated by representative In₂O₃ and nanoscopicorganic dielectrics provides room-temperature fabricated transparentTFTs with performance unobtainable via conventional approaches. Such ahybrid TFT strategy is applicable to other oxide-based TFT structures(bottom source-drain contacts, top gate, etc.), as well as to otherwide-bandgap metal oxide semiconductors and to other transparentultrathin organic dielectrics. Furthermore, these hybrid devices arecompatible with large-scale/large-area deposition techniques and simpledielectric growth processes, and are transparent—a promising approach tohigh-performance, transparent electronics.

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand featuring related to the components/devices and/or methods of thepresent invention including the fabrication of thin film transistordevices comprising various semiconductor and dielectric components, asare available through the fabrication techniques described herein. Incomparison with the prior art, the present components/devices and/ormethods provide results and data which are surprising, unexpected andcontrary thereto. While the utility of this invention is illustratedthrough the use of several components and device configurations, it willbe understood by those skilled in the art that comparable results areobtainable with various other components, devices or configurationsthereof as are commensurate with the scope of this invention.

Example 1

TFT Fabrication. In₂O₃ thin films were deposited on p⁺-Si/SiO₂ (ProcessSpecialties, Inc.), n⁺-Si/(SAS nanodielectric) (n⁺-Si from ProcessSpecialties, Inc.), n⁺-Si/(crosslinked polymer blend dielectric), andPET/ITO/(crosslinked polymer blend dielectric) (ITO/PET,(R_(sheet)=80Ω/□), from CPFilms Inc.), and IAD-derived glass/ITO(Corning 1737F glass substrates from Precision Glass & Optics; the ITOgate was deposited by IAD at room temperature; sheet resistance=60Ω/□)as the back gate. The nanoscopic organic gate dielectrics (SAS, three5.5 nm layers of type III; CPB, 20 nm prepared frompoly-4-vinylphenol+1,6-bis(trichlorosilyl)hexane) were grown vialayer-by-layer self-assembly or spin-coating, as provided in thereferences incorporated herein. Poly-4-vinylphenol and1,6-bis(trichlorosilyl)hexane were purchased from Aldrich and Gelest,respectively.

In₂O₃ films were grown with a Veeco horizontal dual-gun IAD system atroom temperature. The In₂O₃ target (99.99%) was purchased fromPlasmaterials. During the semiconducting In₂O₃ deposition process, thegrowth system pressure and O₂ partial pressure were optimized at4.0×10⁴−4.4×10⁻⁴ torr and 2.2×10⁻⁴−2.6×10⁻⁴ torr, respectively. Thegrowth rate of the In₂O₃ thin films was 3.3±0.2 nm min⁻¹. During theIn₂O₃ drain- and source-electrode deposition, the growth-system pressureand O₂ partial pressure were at 2.7×10⁻⁴ torr and 0.4×10⁻⁴ torr,respectively. The conductivity of the In₂O₃ drain and source electrodeswas measured to be 1,400 S cm⁻¹ by a four-probe technique. The ITO filmswere deposited using the same IAD growth system at room temperature. TheITO target (In₂O₃/SnO₂=9:1) was purchased from Sputtering Materials, andthe ITO growth-process details have been reported elsewhere.

A top-contact electrode architecture was used in TFT device fabrication.The 50 nm Au source and drain electrodes were deposited by thermalevaporation (pressure ˜10⁻⁶ torr) through shadow masks, affordingchannel dimensions of 50/100 μm(L)×5 mm(W). Alternatively, 150 nm In₂O₃source and drain electrodes were deposited by IAD through the sameshadow masks for completely transparent TFTs. The top-contactSi/SiO₂/In₂O₃/Au, Si/SAS/In₂O₃/Au and glass/ITO/SAS/In₂O₃/(Au or In₂O₃)TFT device structures are shown in FIG. 1. Further details concerningorganic dielectric growth and device fabrication are provided in theaforementioned incorporated references and are also reported in theliterature. See, Yoon, M. H.; Facchetti, A.; Marks, T. J. Proc. Natl.Acad. Sci. U.S.A. 2005, 102, 4678; Yoon, M.-H.; Yan, H.; Facchetti, A.;Marks, T. J. J. Am. Chem. Soc. 2005, 127, 10388.

Example 2

Characterization. In₂O₃ film thicknesses were verified using a TencorP-10 step profilometer by etching a step following film growth. XRD θ-2θscans of In₂O₃ were acquired with a Rigaku DMAX-A diffractometer usingNi-filtered Cu Kα radiation. Optical transmittance spectra were acquiredwith a Cary 500 ultraviolet-visible-near-infrared spectrophotometer andwere referenced to the spectrum of uncoated Corning 1737F glass. Filmsurface morphologies were imaged on a Digital Instruments Nanoscope IIIAFM. Quantitative SIMS analysis was carried out on a MATS quadrupoleSIMS instrument using a 15 keV Ga⁺ ion source. Conductivities of thesemiconducting In₂O₃ thin films were measured with a Keithley 2182Ananovoltmeter and 6221 current source. The electrical properties ofhighly conductive ITO and In₂O₃ films were characterized on a Bio-RadHL5500 van der Pauw Hall-effect measurement system. TFT devicecharacterization was carried out on a customized probe station in airwith a Keithley 6430 subfemtometer and a Keithley 2400 source meter,operated by a locally written Labview program and GPIB communication.The parameters μ_(GB) and N_(t) (Table 1) were computed by plottingln(I_(DS)/V_(G)) at a given drain-source voltage using thegrain-boundary trapping model and equation 3, as above.

${I_{DS} = {\frac{W\;\mu_{GB}V_{D}C_{i}V_{G}}{L}{\exp\left( \frac{{- q^{3}}N_{t}^{2}t}{8\; ɛ\;{kTC}_{i}V_{G}} \right)}}},$where W and L are the channel width and length, respectively, μ_(GB) isthe grain-boundary mobility, V_(D) is the applied bias between the drainand source electrodes, C_(i) is the dielectric capacitance, V_(G) is thegate bias, q is the electron charge, N_(t) is the interfacial trapdensity between the semiconductor and dielectric, t is the channelthickness, ∈ is the In₂O₃ permittivity, k is the Boltzmann constant andT is the absolute temperature at room temperature.

Example 3

TFTs fabricated with another inorganic n-channel semiconductor, ZnO, aredemonstrated on inorganic SiO₂ dielectrics. For instance, ZnO films weredeposited on p⁺-Si/SiO₂ (300 nm) substrates at room temperature byion-assisted deposition (IAD). The ZnO thin film growth conditions weresimilar to those of In₂O₃, above. The energies (currents) used toproduce the primary and assisted ion beams were 180 W(186 mA) and 75W(37 mA), respectively. The system growth pressure and O₂ partialpressure were 2.8×10⁻⁴˜3.0×10⁻⁴ Torr and 0.5×10⁻⁴˜0.8×10⁻⁴ Torr,respectively. The growth rate of ZnO thin films was 4.1±0.1 nm/min. AllZnO TFTs were fabricated at room temperature. The ZnO films exhibitedclear field-effect n-type response. Details of such ZnO TFTcharacteristics are shown in FIGS. 13-14. In particular, ZnO TFTsfabricated on p⁺-Si/SiO₂ substrates exhibit typical field-effect I-Vcharacteristics (FIG. 13A) with classical linear and pinch-offsaturation lines. ZnO TFTs show a field-effect mobility of 0.02 cm²/V·sand drain current on/off ratio of ˜10⁴, respectively, with great light-and air-stability when exposed to ambient environment.

While the principles of this invention have been described in connectionwith specific embodiments, it should be understood clearly that thesedescriptions are added only by way of example and are not intended tolimit, in any way, the scope of this invention. For instance, thepresent invention can comprise various silicon gallium-arsenide andother Group II-V semiconductor components. Likewise, the presentinvention contemplates various other device structures, in addition tothin film configurations.

The invention claimed is:
 1. A metal oxide thin film transistor devicecomprising a substrate, a gate conductor, source-drain contacts, and aninorganic-organic hybrid thin film composition comprising an inorganicsemiconductor layer coupled to an organic layer, wherein the inorganicsemiconductor layer is a channel layer and comprises a metal oxidecomprising indium.
 2. The device of claim 1, wherein the inorganicsemiconductor layer comprises a metal oxide comprising indium and one ormore metals independently selected from a Group 12 metal, a Group 13metal, and a Group 14 metal.
 3. The device of claim 1, wherein theorganic layer comprises chemical moieties that can promote substratesorption, condensation, or intermolecular crosslinking.
 4. The device ofclaim 1, wherein the organic layer comprises hydrolyzable moieties. 5.The device of claim 1, wherein the organic layer comprises a polymericcomponent, a π-polarizable component, or a combination thereof.
 6. Thedevice of claim 5, wherein the organic layer comprises a polymericcomponent comprising a polymer selected from poly(vinylphenol),polystyrene, and copolymers thereof.
 7. The device of claim 5, whereinthe organic layer comprises a polymer blend, the polymer blendcomprising a polymeric component and a component promoting substratesorption, condensation, or intermolecular crosslinking.
 8. The device ofclaim 5, wherein the organic layer comprises a π-polarizable componentthat is coupled to at least one of a silyl moiety and a siloxane moiety.9. The device of claim 8, wherein the π-polarizable component comprisesa non-linear optical chromophore.
 10. The device of claim 5, wherein theorganic layer comprises a multi-layered composition, the multi-layeredcomposition comprising periodically alternating layers of differentmaterials.
 11. The device of claim 10, wherein at least some of theperiodically alternating layers are coupled to an adjacent layer by acoupling layer comprising a siloxane matrix.
 12. The device of claim 1,wherein the substrate comprises a transparent material and is coupled tothe inorganic-organic hybrid thin film composition.
 13. The device ofclaim 12, wherein the device is a completely transparent thin filmtransistor device.
 14. The device of claim 1, wherein the substratecomprises a flexible plastic material and is coupled to theinorganic-organic hybrid thin film composition.
 15. The device of claim14, wherein the device is bendable up to a bending radius of curvatureof about 4 cm without residual effects.
 16. The device of claim 1,wherein the inorganic semiconductor layer is physically adhered to theorganic layer.
 17. The device of claim 1, wherein the inorganicsemiconductor layer is physically adhered to the organic layer byadsorption.
 18. The device of claim 1, wherein the organic layer ischemically bonded to the inorganic semiconductor layer.
 19. The deviceof claim 1, wherein the device operates at an operating voltage (V_(G))between about 1 V and about 20 V.
 20. The device of claim 1, wherein thedevice exhibits a field-effect mobility of greater than about 100cm²V⁻¹s⁻¹ at an operating voltage (V_(G)) between about 1 V and about 2V.